This invention relates to an improvement in conventional wet-spinning processes for producing acrylic filaments whereby improved spinning performance (e.g. less filament breaks and wraps) and increased productivity in terms of pounds of filaments per hour are obtained. The term "acrylic", as used herein, means any long chain synthetic fiber forming polymer comprising acrylonitrile units, --CH.sub.2 --CH(CN)--. Acrylic polymers of particular commercial importance are those consisting of at least 85% by weight of acrylonitrile units with the balance comprising either vinylacetate, methyl methacrylate, or vinylacetate and methylvinylpyridine units.
Conventionally, wet-spinning processes for producing acrylic filaments comprise the coupled steps of extruding a solution of acrylic polymer in a suitable solvent therefor (i.e. dope) through one or more spinnerets immersed in a coagulation bath for the polymer to form filaments. Each spinneret typically has at least 20,000 (e.g. 60,000 to 100,000) orifices. The coagulation bath comprises water and solvent for the polymer. Normally, the solvent used in the coagulation bath is the same as that used in the dope. Typical solvents include organic solvents, such as dimethylacetamide (DMAc) and dimethylformamide (DMF), and inorganic solvents, such as an aqueous solution of sodium thiocyanate or nitric acid. The as-spun filaments are withdrawn from the bath, washed with water to remove solvent, wet-stretched several times their as-spun length in a hot (60.degree. to 100.degree. C.) water bath to impart molecular orientation to the filaments and then dried on a drying roll train. The term "drying roll train", as used herein, means a plurality of at least 20 rotatable rolls arranged serpentinely in series over which filaments are passed sequentially from roll to roll and over each roll with a partial wrap and under sufficient tension to prevent slippage of the filaments on the rolls. At least some of the rolls are heated by means of pressurized steam which is circulated internally through the roll(s). The processing conditions (including composition of the dope and coagulation bath, the amount of wet-stretch, temperatures and filament speeds) are correlated to provide useful filaments of a desired denier. Following the drying step, the filaments are further processed in a conventional manner, for example, crimped, annealed and cut to staple or collected as tow.
In the past, acrylic filaments were typically produced in the denier range of 3 to 18 denier and, accordingly, the diameter of the spinneret orifices of existing conventional wet-spinning processes are of an appropriate size to produce filaments having such deniers. However, recently, finer dpf acrylic filaments have gained importance. Unfortunately, such finer denier filaments cannot be economically and feasibly produced using the existing wet-spinning equipment. For example, attempts to provide finer deniers by increasing the wet-stretch imparted to the filaments significantly reduces the spinning performance of the process. On the other hand, replacing the existing spinnerets with spinnerets having smaller orifices is costly. Also, attempts to provide the finer denier filaments by hot-stretching the filaments, for example, on the last rolls of the drying roll train has not proven successful because the filaments cannot be heated to a sufficient temperature on the drying rolls to endure the drying roll stretch, that is, filaments break and the spinning performance of the process is reduced. (Finer dpf filaments have more surface area per pound than larger dpf filaments and require more drying). While more drying rolls could be added, doing so, even if space permitted, would add considerably to the overall cost of the process. Also, for this same reason, the productivity of existing equipment is limited because of its drying capacity, that is, if more pounds of filaments are added to the drying rolls or if the rate, in terms of pounds per hour, at which the filaments are processed is increased, the existing drying roll train simply cannot dry the filaments.